Recently, from the standpoint of global environmental conservation, regulations on substances destroying the ozone layer are being fortified, and among them, as for chlorofluorocarbons (CFCs) known to have a particularly strong destructive force, total disuse was decided at the end of 1995. At the same time, as for hydrochlorofluorocarbons (HCFCs) relatively small in destructive force, regulation of total emission started in 1996, and total disuse in future was decided. In this background, refrigerants to replace CFCs and HCFCs are being developed. It is, accordingly, proposed to use hydrofluorocarbons (HFCs) which do not destroy the ozone layer, but as far as known so far, there is no HFC that can be used alone to replace the HCFCs being presently used in the refrigerating machine and air-conditioner. Therefore, a non-azeotropic mixed refrigerant mixing two or more HFC refrigerants is most highly expected. In particular, a mixed refrigerant of HFC-32 and HFC-125 is a most promising candidate as a substitute refrigerant for HCFC-22 (hereinafter called R22). One of its representative examples is R410A (HFC-32/125=50/50 wt. %).
FIG. 8 is a characteristic diagram showing effects of ratio of charging refrigerant of R22 or R410A on the temperature of compressor coil in a conventional refrigerating apparatus. The ratio of charging refrigerant refers to the ratio of the actual refrigerant amount to the specified refrigerant amount of the refrigerating machine. As known from FIG. 8, when the refrigerating machine or air-conditioner using conventional R22 runs short of refrigerant, along with elevation of compression ratio, the discharge temperature hikes, and the circulation of the refrigerant drops. As a result, the cooling effect declines, and the temperature of the compressor coil elevates. The shaded area in the diagram refers to an example of compressor stopping point by a compressor overload protective device of a small-sized room air-conditioner mounting a constant speed compressor. Considering this example, it is known that the compressor stops when the ratio of charging refrigerant is about 70% in the refrigerating apparatus using R22, that is, when a refrigerant leak of about 30% occurs. (it must be noted, however, this ratio varies somewhat depending on the type of the overload protective device and air-conditioning load.) Therefore, in a refrigerating apparatus using R22, when a refrigerant leak occurs, the compressor overload protective device is actuated by elevation of discharge temperature. It was therefore possible to detect a refrigerant leak early indirectly.
In FIG. 8, however, when running short of refrigerant R410A, rise of discharge temperature of compressor coil is smaller than that in the case of R22, and the cooling effect is enhanced by increase of circulation of refrigerant R410A. Accordingly, it is lower than the discharge temperature R22 of the compressor coil when using R410A. This discharge temperature characteristic of the compressor coil in the event of shortage of refrigerant R410A is a feature of a mixed refrigerant of HFC-32/125. As seen therefrom, when an overload protective device of compressor for R22 machine is used in the refrigerating apparatus using R410A, the compressor can operate in a range of up to the ratio of charging refrigerant R410 of about 30%. Hence, as far as the user does not notice shortage of capacity due to insufficient refrigerant, continuous operation may be executed for a long time.
Methods for detecting shortage of refrigerant amount are disclosed in Japanese Laid-out Patents 62-158966, 1-107070, and 6-137725.
In Japanese Laid-out Patent 62-158966, the outlet temperature and intermediate temperature of a heat exchanger are compared and calculated, and excess or shortage or leak of refrigerant is detected.
It involves the following problems. FIG. 9 is a side view of a heat exchanger in a prior art. As shown in FIG. 9, in a heat exchanger 80, there are plural fins 6 between side boards 7, and a heat transfer conduit 5 and U-pipes 32 to 40 penetrate through the fins 6. Refrigerant enters from an inlet 31, and is discharged from an outlet 41. A second temperature detector 21 for detecting the refrigerant temperature in the heat exchanger is provided in a middle part of the heat exchanger.
In a method for detecting the temperature at the outlet 40 of the heat exchanger and the temperature in the middle part 36, since a differential temperature of .DELTA.T occurs at the ratio of charging refrigerant of about 40 to 70%, refrigerant leak can be detected, but the differential temperature .DELTA.T decreases at about 40%, and refrigerant leak cannot be detected.
In Japanese Laid-out Patent 1-107070, on the other hand, in addition to the differential temperature at the inlet and outlet of refrigerant in the heat exchanger, the differential temperature at the inlet and outlet of the air side is also included in the operation to detect shortage of refrigerant and leak of refrigerant.
However, in the method of detecting the differential temperature of the inlet and outlet of refrigerant, the refrigerant temperature at the evaporator inlet drops suddenly along with decline of suction pressure due to shortage of refrigerant, and hence it is not effective for detection of refrigerant leak. Moreover, these methods require two or more sensors for detecting temperature in the evaporator, and the cost is increased.
Or, in the method of detecting the inlet and outlet temperature at the air side, it also adds to the cost because a temperature detecting sensor is needed in the blow-out part of the indoor unit side.
In Japanese Laid-out Patent 6-137725, meanwhile, the refrigerant temperature in the refrigeration system is detected at specific time intervals, and the refrigerant leak is judged from its changing amount.
This method is, however, constituted to detect the refrigerant temperature in the refrigeration system at specific time intervals, and judge the refrigerant leak by the changing amount of the superheat, and accordingly, same as in the method of detecting the differential temperature at the refrigerant inlet and outlet, capacity drop of evaporator due to refrigerant shortage cannot be detected precisely. In this method, yet, since the changing amount of the refrigerant temperature in the refrigeration system is always stored in order to judge refrigerant leak, the operation is complicated.